Such capsules or micromatrix bodies are used for a multitude of purposes. They serve to hold dyes, inks, chemical agents, pharmaceutically active substances, flavorings, fungicides, bactericides, herbicides, insecticides and other substances. These materials are located in the interior of the capsules or in the matrix body, optionally in dissolved, emulsified or suspended form. The enclosed material is protected by the sheath against the action of light, oxygen and moisture. The active ingredient can be released by diffusion or by mechanical destruction of the capsule wall or the matrix body.
Various methods are known for enveloping or encapsulating hydrophobic liquids. They are described in an easily surveyed form in the journal, Angewandte Chemie, 87, 556-567 (1975).
In most instances, the method of coacervation, as described for example, in Austrian Pat. Nos. 324,281 and 352,686, is used to form the capsules. This method involves the separation of a sol of one or several polymers into a polymer-rich liquid phase and a polymer-poor liquid phase. The coacervate appears first as a fine dispersion of microscopically small droplets of the polymer. If materials are present which are extraneous to the dispersion, the coacervate encloses these materials and encapsulates them. The enveloping polymer layer can then be solidified in a suitable manner.
Coacervation is a suitable method for encapsulating small amounts of substances. Since large volumes are required for the encapsulation, for example, 35 to 50 tons of reaction medium for 1 ton of pesticides, the method of coacervation of large amounts of substances is expensive and cumbersome and less suitable.
U.S. Pat. No. 3,557,515 discloses envelopment by interfacial polycondensation as a further method of encapsulation. In this method which may be carried out continuously, a first reaction participant and a reaction partner, complementary to the first reaction participant, are required. The two materials are present in different phases so that, after the formation of a dispersion, the two reaction partners react with one another at the phase boundary of the individual droplets. The droplets are then enveloped in this manner. It is difficult to control the reaction in this method and a high degree of technical effort and expenditure is required.
To avoid the necessity of two reaction partners, the production of such microcapsules from polyurea has been attempted. Pursuant to British Pat. No. 1 371 179, a polyisocyanate is thus mixed with the substance to be encapsulated with formation of an organic phase. Subsequently, this oily phase is emulsified in an aqueous phase. After obtaining an emulsion with the desired particle size of the emulsified phase, the interfacial polymerization, that is, the reaction of the isocyanate groups with water, is started by addition of a catalyst and/or by increasing the temperature. After the reaction, discrete capsules are present in the form of an aqueous suspension. This method has the disadvantage that carbon dioxide is evolved during the hydrolysis of the isocyanate which causes foaming of the reaction medium. Moreover, in the course of storage, carbon dioxide is gradually released from the unreacted, residual isocyanate groups, as a result of which the wrapping of the microcapsules produced in this manner may burst.
Pursuant to U.S. Pat. No. 4,406,741, it is possible to subject the microcapsules to an aftertreatment with ammonia or amines to reduce the content of residual isocyanate groups. However, this requires an additional step in the method which is not justifiable, especially for economic reasons.
In German Offenlegungsschrift No. 1 962 039, a further method is described for encapsulating particles of a phase dispersed in water by formation of a solidifiable boundary layer about the particles. In this method, surface active substances which contain at least one silyl group with at least one terminal hydrolyzable group linked to the silicon atom, are added during or after the preparation of the dispersion and the compounds, optionally in the presence of an alkoxysilane, are hydrolyzed and condensed. This method is suitable for stabilizing emulsions, especially silicone oil/water emulsions. However, it does not impart the enveloped oil droplets with sufficient stability to resist mechanical influences.
U.S. Pat. No. 3,946,106 discloses a method for the preparation of pharmaceutical formulations with a controlled release of the active ingredient. In this method, the active substance is dissolved in the desired concentration, generally up to the saturation limit, in an aqueous solution of polyethylene glycol. Subsequently, a silicone polymer which can be vulcanized at room temperature is added. This mixture is emulsified by vigorous stirring. After the addition of a suitable catalyst, the emulsion is filled into a silicone tube in which the vulcanization takes place within an hour. After removal of the silicone tube, the vulcanized material is present in the form of a string matrix which, by cutting off fragments of different lengths, contains different amounts of active ingredient.
In this method, no microcapsules are thus formed, but rather relatively large matrix bodies. These cannot be comminuted, let alone, sprayed.
There exist a large number of other publications which deal with this problem, but which do not go beyond the state of the art shown above.